1. Field of the Invention
This invention pertains to communication systems and, more particularly, to communications systems wherein a signal is rendered unintelligible during transmission in order to insure privacy.
The communication of confidential information is generally accomplished by providing, at each terminal station of a system, suitable equipment for transforming applied message waves into an unintelligible form before transmission over the medium connecting the terminal stations. Unauthorized personnel are therefore prevented from detecting transmitted signals and thereby obtaining an understanding of the information transmitted.
2. Description of the Prior Art
Diverse systems have been devised in which electrical signals corresponding to speech or other signals, to be privately transmitted, have been rendered unintelligible. For example, speech signals have been "scrambled," or "garbled," in the time or frequency domain or both; their frequency components have been inverted with respect to a selected nominal frequency or such signals have been broken up into segments which are then transmitted in alternation with corresponding segments of another message.
Time scrambling, i.e., garbling a signal in the time domain, usually introduces an excessive amount of transmission delay and discontinuities in the scrambled signal which result in an intolerable level of noise in the recovered signal. Systems which scramble in both time and frequency, simultaneously, generally suffer from the same detrimental behavior. Frequency scrambling systems, on the other hand, have negligible transmission delay and a greatly improved recovered quality. However, such systems, in order to attain a high degree of privacy, have relied upon complicated and intricate arrangements of apparatus and equipment. The cost and complexity of such equipment has therefore rendered it substantially impractical for use in a commercial environment, where lost cost and small weight and size are essential.